A linear model for the structure of turbulence beneath surface water waves

The structure of turbulence in the ocean surface layer is investigated using a simplified semi-analytical model based on rapid-distortion theory. In this model, which is linear with respect to the turbulence, the flow comprises a mean Eulerian shear current, the Stokes drift of an irrotational surface wave, which accounts for the irreversible effect of the waves on the turbulence, and the turbulence itself, whose time evolution is calculated. By analysing the equations of motion used in the model, which are linearised versions of the Craik–Leibovich equations containing a ‘vortex force’, it is found that a flow including mean shear and a Stokes drift is formally equivalent to a flow including mean shear and rotation. In particular, Craik and Leibovich’s condition for the linear instability of the first kind of flow is equivalent to Bradshaw’s condition for the linear instability of the second. However, the present study goes beyond linear stability analyses by considering flow disturbances of finite amplitude, which allows calculating turbulence statistics and addressing cases where the linear stability is neutral. Results from the model show that the turbulence displays a structure with a continuous variation of the anisotropy and elongation, ranging from streaky structures, for distortion by shear only, to streamwise vortices resembling Langmuir circulations, for distortion by Stokes drift only. The TKE grows faster for distortion by a shear and a Stokes drift gradient with the same sign (a situation relevant to wind waves), but the turbulence is more isotropic in that case (which is linearly unstable to Langmuir circulations)

Linear Invariant Systems Theory for Signal Enhancement

This paper discusses a linear time invariant (LTI) systems approach to signal enhancement via projective subspace techniques. It provides closed form expressions for the frequency response of data adaptive finite impulse response eigenfilters. An illustrative example using speech enhancement is also presented.Este artigo apresenta a aplicação da teoria de sistemas lineares invariantes no tempo (LTI) na análise de técnicas de sub-espaço. A resposta em frequência dos filtros resultantes da decomposição em valores singulares é obtida aplicando as propriedades dos sistemas LTI

Purification of propylene using functionalized inorganic membranes and membrane reactors

Tese de doutoramento. Chemical and Biological Engineering. Universidade do Porto. Faculdade de Engenharia. 201

Valuing nested names in the Portuguese olive oil market: An exploratory study

The Portuguese olive oil market had a remarkable development in recent years. Production is rising steadily is response to a EU program supporting a renewal of olive groves. Moreover there is a proliferation of national brands and private labels. These are often associated to regional collective labels or to organic production. The aim of our research is to determine how consumers value these nested names or co-brands. We conducted a pilot survey on a convenience sample of 103 consumers in the Oporto and Lisbon metropolitan areas as well in a rural area. Our results reveal some contradictions, for instance while origin is an important purchasing criteria, few PDO olive oils are recognized. Moreover, only 25% of respondents identify organic olive oils sold in the market and this attribute is one of the last purchasing criteria, but organic olive oils have the highest willingness to pay. Finally we find that associating a PDO to private labels increases willingness to pay by 33.3%, but doesn’t affect valuation of national brands. While we can’t take definite conclusions our findings give us interesting cues for future research. Therefore we aim to investigate whether regional identity, alternative usage and health or environmental conscience determine of affect valuation and choices of different olive oils brands and labels.olive oil, nested names, valuation., Agribusiness, Agricultural and Food Policy, Community/Rural/Urban Development, Food Consumption/Nutrition/Food Safety, Labor and Human Capital,

Análise exploratória espacial das atividades económicas em Portugal

Mestrado em EconomiaAs atividades económicas não se distribuem uniformemente em todo o território. Como tal, a concentração geográfica das atividades económicas tem despertado um grande interesse na comunidade científica, na sequência de exemplos famosos como o Silicon Valley (Califórnia), Route 128 (Boston), Cambridge (Reino Unido), o estado federal de Baden Wurttemberg (Alemanha). Dada a importância desta matéria, geralmente aceite como uma prioridade no quadro das diversas políticas de desenvolvimento económico, o objetivo deste trabalho é medir e descrever o padrão de distribuição espacial dos principais sectores da atividade económica em Portugal. Para isso, seguimos a metodologia de R. Guillain and J. Le Gallo (2010), combinando o coeficiente de Gini locacional com uma Análise Espacial de Dados Exploratória, aplicada aos dados do emprego por sector e por municípios em 2009 e 2010. Esta abordagem tem a vantagem de introduzir uma dimensão espacial nas medidas habituais de concentração, procurando assim determinar o padrão de localização de cada sector de atividade e medir a correlação espacial.Economic activities are not evenly distributed throughout the territory. As such, the geographical concentration of economic activities has aroused a great interest in the academic community, following such famous examples as Silicon Valley (California), Route 128 (Boston), Cambridge (UK), the federal state of Baden Wurttemberg (Germany). Given the importance of this matter, regarded as a priority in terms of economic development policies, the aim of this paper is to measure and describe the spatial distribution pattern of the main sectors of economic activity in Portugal. For this we follow the methodology of R. Guillain and J. Le Gallo (2010), combining the locational Gini coefficient with an Exploratory Spatial Data Analysis, applied to the employment data by sector and by municipalities in 2009 and 2010. This approach has the advantage of introducing a spatial dimension to the usual measures of concentration, thus seeking to determine the location pattern of each sector of activity and to measure spatial correlation

Diagnosing lee wave rotor onset using a linear model including a boundary layer

A linear model is used to diagnose the onset of rotors in flow over 2D hills, for atmospheres that are neutrally stratified near the surface and stably stratified aloft, with a sharp temperature inversion in between, where trapped lee waves may propagate. This is achieved by coupling an inviscid two-layer mountain-wave model and a bulk boundary-layer model. The full model shows some ability to diagnose flow stagnation associated with rotors as a function of key input parameters, such as the Froude number and the height of the inversion, in numerical simulations and laboratory experiments carried out by previous authors. While calculations including only the effects of mean flow attenuation and velocity perturbation amplification within the surface layer represent flow stagnation fairly well in the more non-hydrostatic cases, only the full model, taking into account the feedback of the surface layer on the inviscid flow, satisfactorily predicts flow stagnation in the most hydrostatic case, although the corresponding condition is unable to discriminate between rotors and hydraulic jumps. Versions of the model not including this feedback severely underestimate the amplitude of trapped lee waves in that case, where the Fourier transform of the hill has zeros, showing that those waves are not forced directly by the orography